Inorganic materials derived from the sol-gel process usually show excellent mechanical properties from the viewpoint of abrasion and wear resistance. This behaviour is connected with a high surface hardness accompanied by a high elastic modulus which is a measure for the stiffness, also in the densified material. One disadvantage of this type of materials is the required high processing temperature of about 450° C.-500° C. in combination with long processing times in order to obtain completely densified layers or moulded articles. This complete densification is an important step to obtain a high elastic modulus together with sufficient strength. Another disadvantage is the high volume shrinkage which occurs during densification which may lead to residual stresses in the materials in many cases. Therefore the maximum layer thickness which can be achieved without occurrence of cracks in the layers is usually limited to only a few micrometers if reasonable curing times are considered and for this reason it is almost impossible to obtain patterns with a high aspect ratio.
On the other hand, layers or even foils with a thickness in the millimeter range without cracks can be produced from materials based on organic polymers such as e.g. polyimides due to their higher relaxation ability compared to the sol-gel derived inorganic materials. Besides a high strength the pristine polyimide type polymers also show a quite high elastic modulus in the range of 5 GPa which is caused by their structure along the polymer chain mainly consisting of aromatic and aromatic-aliphatic group containing monomer units. On the other hand, such excellent mechanical behaviour of the polyimides can only be reached when a densification temperature of about 300° C. is applied, which is still quite high.
When polymerizable groups such as e.g. unsaturated carbon-carbon double bonds are introduced in such systems to obtain patternability, the required curing temperature usually decreases, but in all cases the mechanical properties decrease significantly at the same time. Another disadvantage of polymer systems having aromatic groups in the structure is that they are not completely colourless because of light absorption of the conjugated double bond systems in the visible range, thus limiting the obtainable aspect ratio to some extent if fine-patterning processes like photolithography are involved.
Several approaches have been followed in the past to overcome the drawbacks of both pure inorganic and organic polycondensates mentioned above wherein inorganic structures with intrinsic high elastic modulus and organic radiation curable structural units have been combined. Nanoparticles have been used to provide inorganic rigid phases in a softer and photopolymerizable matrix in order to avoid light scattering and maintain transparency. JP-A-2005015581, JP-A-2005089697 and JP-A-2000347001 describe compositions comprising organic or organic-inorganic photopolymerizable polymers having epoxy groups which may also contain inorganic particles. These compositions have been used for transparent hard coatings. On the other hand, patterned structures are not described.
Organic matrices derived from polymers filled with rather high loadings (e.g. up to 30 vol. %) of inorganic nanoparticles have been prepared. For example, EP-A-0819151 describes such matrices for transparent composite adhesives. A photo-patterning process such as photolithography requires shape stability as well as chemical resistance, if patterns with high aspect ratio are desired. However, the limited resistance of organic matrices against organic solvents would cause undesired swelling or even dissolution of the matrix. Moreover, the viscous flow behaviour is directly connected to the viscous behaviour of the monomers used and usually results in insufficient shape stability of created patterns if the processing temperature is increased.
As a consequence, fine patterning with an aspect ratio >1 can usually not be achieved with such organic matrices due to undesired flow and swelling behaviour which reduces patterning accuracy and the final mechanical properties. For these reasons patterning is also not described for such types of systems.
EP-A-0991673, WO-A-98151747 and JP-A-2005004052 describe photocurable systems based on methacrylate or methacrylate hybrid matrices which can be patterned with reasonable aspect ratios in the range of 1 (height/width) using photolithographic techniques. On the other hand, these systems suffer from a limited chemical stability due to the possible hydrolytic attack on the ester bonds by bases, acids or organic solvents and limited temperature stability. These drawbacks limit their usability for microelectronic devices for which a high stability is required in many fabrication processes.
DE-A-10323729 describe organic-inorganic nanocomposites consisting of hybrid polymer-like matrix showing thixotropy which is a well-known viscosity property. Said systems may contain inorganic particles in order to control the viscosity behaviour. These thixoptropic systems are suitable for mechanical patterning processes such as embossing with a stamp. The disadvantage is that even at high degrees of filling with nanoparticles the mechanical properties of the cured nanocomposite systems are mainly dominated by the matrix and a rather weak interface, because the interaction forces between particle surface and matrix are too low to allow the reversible flow behaviour related to thixotropy and therefore effective stress transfer from the matrix to the incorporated inorganic nanophases is impeded.
DE-A-102005002960 describes a composite composition for micropatterned layers comprising a hydrolysate or condensate of organosilanes, an organic compound having at least 2 epoxy groups and a cationic initiator. With said composite compositions improved patterned structures can be obtained. However, for some applications an improved elastic modulus of the material is desirable, in particular if the system is cured under mild conditions.